Abstracf-Energy storage at a groove edge, or similar discontinuity in the substrate surface, produces a phase shift in a surface acoustic wave incident on the discontinuitg. Such phase shifts are a major source of phase errors in surface-wave devices employing etched grooves or deposited stripes on the substrate surface. An improved equivalent network, in which this stored energy is represented as a simple shunt susceptence, is presented for a Rayleigh wave normally incident on a step discontinuity on the substrate surface. With this network second-order effects observed at the fundamental frequency of a uniform array of shallow grooves can now be predicted. The presence of stored energy also exerts a profound influence on the wideband harmonic response of the array. In particular, a strong response is predicted at even harmonics of a grating the groove width of which is half the grating period, in contradiction with simpler theories. The validitg of this prediction is verified by measurements at the second harmonic, with excellent agreement between theory and experiment.

The Influence of Stored Energy at Step Discontinuities on the Behavior of Surface-Wave Gratings

The use of reflective arrays in surface-wave devices has yielded new devices such as resonators in addition to an enlarged parameter range for devices such as radar pulse expanders and compressors. A number of different grating geometries, substrates, and types of reflectors have been employed in resonators, band-pass filters, filter banks, oscillators, and dispersive delay lines. Simple, yet accurate, models have been developed for design and analysis of reflection gratings in these applications. The major application of reflective arrays has been in radar pulse compressors wherein low spurious levels and precise phase and amplitude response have been achieved in devices with time-bandwidth products as large as 104. Other signal-processing applications such as chirp-z transforms can exploit the large time-bandwidth products available in reflective-array devices.

Properties and applications of reflective-array devices

This paper investigates the combination of chirp modulation and pseudonoise phase shift keying for spread-spectrum transmission of digital data. Data encoding is accomplished by assignment of positive and negative dispersive slope chirps for binary ones and zeros. Each chirp is then modulated by one period of a PN signature sequence. The performance in additive white Gaussian noise is analyzed, measures of performance being mean acquisition time, mean hold-in time, and bit error probability. Calculated and experimental results are presented for a 105 kbit/s modem. In the test system, SAW dispersive delay lines with a bandwidth of 16.4 MHz and 9.5 μs dispersion time serve for the key operations of chirp generation and pulse compression, and the signature codes are 127 bit m -sequences.

A Spread-Spectrum Concept Combining Chirp Modulation and Pseudonoise Coding

With the proliferation of Internet-of-Things (IoT), the chirp spread spectrum (CSS) technique is re-emerging for communications. Although CSS can offer high processing gain, its poor spectral efficiency and the lack of orthogonality among different chirps tend to limit its potential. In this paper, we derive the condition to orthogonally multiplex an arbitrary number of linear chirps. For the first time in the literature, we show that the maximum modulation rate of the linear continuous-time chirps satisfying the orthogonality condition can approach the Nyquist signaling rate, the same as single-carrier waveforms with Nyquist signaling or orthogonal frequency-division multiplexing signals. The performance of the proposed orthogonal CSS is analyzed in comparison to the emerging LoRa systems for IoT applications with power constraint, and its capability for high-speed communications is also demonstrated in the sense of Nyquist signaling.

E. Young

Papers

Application of Chirp SWD for Spread Spectrum Communications